This invention relates generally to optical position detection systems and, more particularly, to autonomous docking systems for spacecraft. Docking one space vehicle with another is typically accomplished with the help of a navigational computer, which controls thrusters on one of the vehicles, which in turn control the position and attitude of the vehicle in three-dimensional space. Although docking might be accomplished by controlling both vehicles, for purposes of explanation it will be assumed that one of the vehicles is not controlled in position or attitude during the docking maneuver. It is further assumed that this vehicle has been stabilized in attitude such that it is not spinning and is in a docking attitude with respect to the other approaching vehicle. The navigational computer in the approaching vehicle depends for its operation on having accurate input data defining the position and relative orientation of the other vehicle. The invention is concerned with a new approach to providing these data.
Optical sensor systems typically use a reflective "target" affixed to the vehicle being approached. To distinguish the target from other nearby reflective objects, the target is responsive only to light of a selected wavelength, for example blue light. Unfortunately, reflections from other portions of the spacecraft also occur when a blue light source is used to detect the target. Elimination of "clutter" in the resulting image is effected by alternately illuminating the vehicle with blue light and light of another wavelength, such as red light. The red light is not reflected by the target but, in general, the other reflective portions of the vehicle reflect both red and blue light equally well. The image obtained from the red light is then subtracted from the image obtained from the blue light, thereby eliminating the clutter, at least in theory.
The principal difficulty with this approach is that the clutter components of the two images are generally not identical, because the non-target components vary with the relative positions of the vehicles. Consequently, even after differencing of the two images, multiple dots remain and some type of image processing algorithm is needed to help distinguish the target dot from the other dots in the image. The shape of the target is usually of no help in this regard until the distance between the vehicles is quite small.
It will be appreciated from the foregoing that there is still a need for improvement in autonomous spacecraft docking procedures, and in particular in the optical sensing systems used to generate position and attitude data for the navigational computer. The present invention satisfies this need.